Insights into the microstructure and load-dependent wear characteristics of the boride layer on Inconel 718 alloy

Dense compact boride layer was developed on the Inconel 718 alloy by powder pack-boriding process. The microstructure and load-dependent wear characteristics of the boride layers were systematically investigated. It is found that the boride layer on Inconel 718 alloy consists of a ∼2.0 $\mu$m thick nickel-rich top layer, a ∼11.3 $\mu$m thick compound layer with nanocrystalline (Ni,Fe)₂₃B₆ and (Cr,Fe)B grains, and a ∼10.9 $\mu$m thick diffusion layer with dendritic Cr₂B precipitation within the matrix. The phase evolution of the boronizing layer are explainable by thermodynamic modeling of Ni-Cr-B system and the phase stability for metastable Ni-B compounds is confirmed by DFT simulation. The boride layer on Inconel 718 alloy displays good wear resistance against Si₃N₄ ceramic balls both at room temperature and 500 ℃. Specifically, the wear rate at room temperature is in the order of 10^-6 mm³·N^-1 m^-1 and the wear mechanism is combined adhesive/abrasive wear and oxidation wear. A higher loading is found to promote the formation of boron oxide tribolayer on the friction ball surface, which mitigates the wear loss effectively. At elevated temperature, the wear rate is in the order of 10^-4 mm³·N^-1 m^-1 and the wear mechanism is severe abrasion wear and oxidative wear. The employment of higher loads is beneficial to suppressing the formation of nickel oxide debris, which participates the friction process as third body and enhances wear loss.